1. Field of the Invention
This invention generally relates to the field of signal detection. More particularly, the present invention relates to a method and device that detects the signal on a defect disc.
2. Description of the Prior Art
Nowadays, disc-type storage media are broadly used in keeping data due to their storage capacity. Such disc-type storage media like optical discs, i.e. CD-R discs, CD-RW discs, DVD-R discs, DVD-RW discs, DVD+R discs, DVD+RW discs, or DVD-RAM discs etc., also provide better protection to the data stored on them against damage. However, these characteristics mentioned above do not mean the optical discs are faultless storage media for storing data because some defects might take place on their bare surfaces. For example, a deep scratch, a shallow scratch, and even a fingerprint. These defects could result in not only reading or writing errors but also a system disturbance while the system reads or writes data. Hence, it is an important thing to detect existing defects for protecting the system from a disturbed or instable situation.
It is well known to use the difference of signal amplitude, such as an RF level (RFLVL) or a sub-beam added (SBAD) signal, to detect an existing defect. As shown in FIG. 1A, a defect detection applying the RFLVL is illustrated. An RF signal 110 has a hollow region 112 in a time period 120. That means the corresponding data of the hollow region 112 is damaged by a defect, so that the RF signal 110 in the time period 120 cannot be read out. Further, the depth of the hollow region 112 represents the depth of the defect. An RFLVL signal 114, which is formed from the RF signal 110 passing a low pass filter, shows the envelope of the RF signal 110. A detection threshold 130 is a fixed DC referred voltage level. As the RFLVL signal 114 is lower than the detection threshold 130 in the time period 120, a defect flag signal 140 is raised from “0” to “1”. Moreover, a FE/TE signal 150 respectively generates a positive surge 152 and a negative surge 154 at the beginning and the end of the time period 120 to indicate a focusing and a tracking error signal. However, while the defect flag signal 140 is set from “0” to “1”, a servo system, such as a focusing and a tracking servo, and a data path control system, such as a preamplifier, a slicer, or a phase lock loop (PLL), can reduce the potential disturbance and instability through applying some appropriately protective methods and devices.
Referring to FIG. 1B, an RF signal 110-1 has a hollow region 112-1 in a time period 120-1. That also means the corresponding data of the hollow region 112-1 is damaged by a defect, so that the RF signal 110-1 in the time period 120-1 cannot be totally read out. But, the depth of the hollow region 112-1 is not deep as the hollow region 112 shown in FIG. 1A, since it might just result from a shallow defect, such as a shallow scratch. An RFLVL signal 114-1 shows the envelope of the RF signal 110-1. A detection threshold 130-1 is a fixed DC referred voltage level like the detection threshold 130 shown in FIG. 1A. Obviously, the RFLVL signal 114-1 is always higher than the detection threshold 130-1 because the shallow defect does not make the hollow region 112-1 deep enough. Hence, not only a defect flag signal 140-1 has no response to the shallow defect, but also a FE/TE signal 150-1 has no apparently change except a little noise. Furthermore, since the shallow defect is not detected, some protective methods and devices are not triggered to protect the system from the potential disturbance and instability. In other words, the servo systems and the data path control systems are easily affected by the disturbance and instability in this defect situation.
Similarly, referring to FIG. 1C, an RF signal 110-2 has a hollow region 112-2 in a time period 120-2. That means the corresponding data of the hollow region 112-2 is slightly affected by a defect, so that the RF signal 110-2 in the time period 120-2 has weaker amplitudes. Also, the depth of the hollow region 112-2 is not deep like the hollow region 112-1 shown in FIG. 1B, since it might only result from a shallow defect, such as a fingerprint. An RFLVL signal 114-2 shows the envelope of the RF signal 110-2 and a detection threshold 130-2 is a fixed DC referred voltage level like the detection threshold 130 shown in FIG. 1A. The RFLVL signal 114-2 is always higher than the detection threshold 130-2 in this defect situation, because the shallow defect does not make the hollow region 112-2 deep enough. Thus, not only a defect flag signal 140-2 has no response to the shallow defect, but also a FE/TE signal 150-2 has no apparently change except a little noise. This situation is similar to the situation described in FIG. 1B; the servo systems and the data path control systems cannot be safely protected. On the other hand, however, the defects shown in FIG. 1B and FIG. 1C further include different statuses according to their damaged depth, width and direction; some defects might still have original data, but others have only destroyed data. Therefore, it is difficult to determine the defect flag signal simply by the detection threshold comparison.
In view of the drawbacks mentioned with the prior art of defect signal detection, there is a continued need to develop a new and improved method and device that overcomes the disadvantages associated with the prior art of defect signal detection. The advantages of this invention are that it solves the problems mentioned above.